1. Field of the Invention
The present invention relates to an optical scanner using a rotary polygon mirror.
2. Description of the Related Art
In an optical scanner, an approximately parallel light beam from a light source device is focused and formed by a cylindrical lens as a linear image extending in a main scanning corresponding direction and is deflected by a rotary polygon mirror having a deflection-reflecting face in the vicinity of an image forming position of this linear image. The deflected light beam is formed as an image in the shape of a light spot on a scanned face by an image forming optical system to perform an optical scanning operation. This optical scanner is well known in association with an optical printer, a laser facsimile, a digital copying machine, a laser photoengraving machine, etc.
As is well known, in the optical scanner using the rotary polygon mirror, there is a problem about the inclination of a reflecting face. To correct the inclination of the reflecting face in the optical scanner, the approximately parallel light beam from the light source device is focused and formed by the cylindrical lens as the linear image extending in the main scanning corresponding direction in the vicinity of the deflection-reflecting face of the rotary polygon mirror. A start point of the deflection by the deflection-reflecting face and the scanned face are arranged by the image forming optical system in an approximately conjugate relation in geometrical optics with respect to a secondary scanning direction.
Therefore, the image forming optical system becomes an anamorphic optical system having a strong power in the secondary scanning direction in comparison with that in the main scanning direction. Accordingly, when a curvature of a field in the main scanning direction is preferably corrected, a large field curvature tends to be caused with respect to the secondary scanning direction.
As mentioned above, the curvature of a field in the secondary scanning direction causes the diameter of the light spot for scanning the scanned face in the secondary scanning direction to change together with the position of the formed image. Further, the diameter of the light spot in the secondary scanning direction becomes uneven in a main scanning region, which is a great obstacle in realizing an optical scanning operation of high density.
There are various kinds of trials for correcting the above field curvature in the secondary scanning direction by a performance of the image forming optical system. However, these trials have the problems that it is very difficult to design the image forming optical system so as to satisfy the requirement of high density.